/usr issue

I find /usr is filled up so quickly.Now it is around 90% full. Is there a way to find out why it is filled up so quickly? And how could i bing it down? Some one suggested that i can delete some log files in usr/local/apache/logs/ or /usr/local/apache/domlogs. But these log files seem related to stats programme, so I do not want to delete them. Is there another way that i can cut down the used space of /usr?